عن عُبَادَةَ بْنِ الصَّامِت رضي الله عنه عن النبي صلى الله عليه وسلم قال: «لا صلاة لمن لم يَقْرَأْ بفاتحة الكتاب». [صحيح] - [متفق عليه] المزيــد ...
‘Ubādah ibn As-Sāmit (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "The prayer of someone who did not recite Sūrat al-Fātihah is invalid." Sahih/Authentic. - [Al-Bukhari and Muslim]
Sūrat al-Fātihah is the mother and spirit of the Qur'an, as it combines all types of praises and sublime attributes of Allah, the Almighty. It also speaks about the dominion and irresistible power of Allah, the Day of Judgment, recompense, worship, and intention. These are various types of monotheism and religious obligations.
Therefore, it is compulsory to recite it in each Rak‘ah, and the validity of prayer is contingent upon its recitation. Thus, from the Shariah perspective, the essence of prayer is negated, if Sūrat al-Fātihah is missing, as confirmed by the Hadīth in which Abu Hurayrah (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: " A prayer is incomplete in which the Mother of the Qur'an (Sūrat al-Fātihah) is not recited."
An exception to that is when someone joins the prayer while the Imām is bowing down. In this particular case, he should make Takbīr of Ihrām and then bow down, without having to recite Sūrat al-Fātihah. This is approved by another Hadīth, and because that person did not catch the time of its recitation (standing position) .